a
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan. E-mail:
[email protected]; Fax: +886 2 23636359; Tel: +886 2 23630231 ext. 3315 b Advance Instrumentation Center, National Taiwan University, Institute of Organic and Materials, National Taipei University of Technology, Taiwan
Communication
Ken-Tsung Wong,*a Tsung Hsi Hung,a Shen C. Kao,b Chung Hsien Choua and Yuhlong Oliver Sua
CHEMCOMM
www.rsc.org/chemcomm
Synthesis and properties of novel bis(triarylamines) based on a 3,3A-diphenyl-2,2A-bithiophene core
Received (in Cambridge, UK) 10th April 2001, Accepted 28th June 2001 First published as an Advance Article on the web 3rd August 2001 An efficient synthesis of 3,3A-diphenyl-2,2A-bithiophene based bis(triarylamines) and their physical properties are reported. Amorphous triarylamines with high glass transition temperatures (Tg) are widely used as hole-transporting materials in organic light-emitting devices (OLED).1 Thiophene and oligothiophene-linked triarylamines and bis(triarylamines), with phenyl rings separating the thiophene system from the terminal diarylamino groups were first introduced by Shirota et al.2 These amorphous materials form thin films with high morphological stability and also show interesting hole-transporting properties, resulting in their successful use as colour-tunable emitting materials for OLED devices. Triarylamines with diarylamino group(s) directly attached to thiophene are one of the least exploited class of hole-transporting materials.3,4 Recently, Watanabe and co-workers5 have accomplished a systematical study on Pd-catalysed amination for the synthesis of 2-diarylaminothiophenes and 2,5-bis(diarylamino)thiophenes. In addition to this, a new synthetic strategy involving transition-metal free cyclization followed by thermal decarboxylation has also been introduced for the synthesis of triarylamines bearing a thiophene moiety.6 However, the physical properties of thiophene based triarylamines are still limited.5,6 In this communication, we report the synthesis and physical properties of a new class of bis(triarylamines) based on 3,3Adiphenyl-2,2A-bithiophene as a central linkage. It is well documented that the physical properties of oligothiophenes and polythiophenes strongly depend on the nature of the substituents. However, the similar oxidation potential and absorption lmax of bithiophene and 3,3A-diphenyl-2,2A-bithiophene have been attributed to counterbalancing the resonance effect of phenyl substituents by the steric effect.7 Interestingly, polythiophenes with phenyl substituent(s) at its 3- and 4-position exhibit higher thermal stability and lower oxidation potentials.8 † Electronic supplementary information (ESI) available: experimental details and NMR spectra. See http://www.rsc.org/suppdata/cc/b1/ b103194p/
Accordingly, the resulting new class of bis(triarylamines) based on 3,3A-diphenyl-2,2A-bithiophene could enhance the thermal and morphological stability, which are crucial for their application in optoelectronic devices. Cu-promoted Ullman reaction for the synthesis of 3,3Adiphenyl-2,2A-bithiophene was first reported by Johnson in 1976.9 Further synthetic efforts based on the Ni-catalysed Kumada coupling reaction of 2-halo-3-phenylthiophene with 2-magnesium-3-phenylthiophene gave 3,3A-diphenyl-2,2A-bithiophene in poor yields.7,8 We report herein a more efficient synthetic pathway (Scheme 1). In the presence of a catalytic amount of Pd(PPh3)4, 3,3A-dibromo-2,2A-bithiophene 110 was treated with phenylboronic acid in DME to afford 3,3Adiphenyl-2,2A-bithiophene (2) in 86% yield. Regioselective bromination11 of 2 was accomplished by treating 2 with bromine in AcOH–CHCl3 (1+2) solution, which afforded 5,5Adibromo-3,3A-diphenyl-2,2A-bithiophene 3 in 95% yield. Six different diarylamines were screened to react with 3 respectively in the presence of a catalytic amount of Pd(OAc)2 and PBut3 in toluene12 at reflux temperature, resulting in compounds 4a–4f as bright yellow solids in moderate yields (Table 1). For a comparative study of the influences of phenyl
Scheme 1 Reagents and conditions: a, PhB(OH)2, Pd(PPh3)4, Na2CO3, DME, reflux 2 d, 86%; b, Br2, AcOH–CHCl3 (1+2), 0 °C to rt 98%; c, diarylamine, Pd(OAc)2, PBut3, NaOBut, toluene, reflux overnight.
Table 1 Chemical yields and physical properties of 5,5A-bis(diarylamino)-3,3A-diphenyl-2,2A-bithiophenes 4 and model compound 5 Compound
Yield (%)a
lmax/nm, log eb
lem/nmc
Epa, Epc /mVd
Tg/°Ce
4a 62 263 (4.59), 367 (4.12) 535 550, 430 55 4b 56 262 (4.69), 365 (4.26) 535 595, 440 85 4c 60 260 (4.96), 317 (4.87) 538 570, 465 83 4d 52 254 (4.90), 363 (4.09) 539 605, 470 114 4e 51 256 (4.99), 376 (4.54) 494 (545, 455), (650, 570) 119 4f 53 265 (4.94), 381 (4.48) 458, 503 600, 500 124 5 62 273 (4.38), 392 (4.42) 480 (460, 385), (695, 610) 67 a Isolated yield by column chromatography on SiO (hexane–CH Cl = 4+1) with satisfactory spectral analyses (1H, 13C, mass, HRMS, and IR). b Recorded 2 2 2 in EtOAc; units of e mol21 dm3 cm21. c Recorded in EtOAc and excited at lmax. d Performed in 0.1 M solution of n-Bu4NPF6 in CH2Cl2, carbon electrode 21 was used as the working electrode, scan rate 100 mV s , Ag/AgCl as reference electrode. e Analyzed by differential scanning calorimetry (DSC), Tg was recorded from heating (10 °C min21) a liquid nitrogen quenched melt-sample.
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Chem. Commun., 2001, 1628–1629 This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b103194p
Fig. 1 Absorption and photoluminescence spectra of 4b (solid), 4e (dot), and 5 (dash-dot) in EtOAc (1.0 3 1025 M)
Fig. 2 Cyclic voltammogram of 4b (solid), 4e (dot), and 5 (dash-dot) in CH2Cl2.
substituents on the physical properties, model compound 5 was also synthesized by the Pd-catalysed amination of 5,5Adibromo-2,2A-bithiophene. The absorption spectra of 4a–4f in ethyl acetate showed similar behaviour, displaying two absorption maxima (Table 1). The second bands of 4e and 4f were slightly red-shifted compared to that of 4a–4d. Compound 4a–4d were highly fluorescent. These results indicate that the absorption lmax is relatively insensitive to the nature of the diarylamino substitutents. However, the photoluminescence efficiency and emission maxima are strongly dependent on the structural feature of the terminal diarylamino groups. Thus, bis(triarylamines) (4a–4d) with less conjugated diarylamino substituents showed more efficient photoluminescence than when compared with those bearing more conjugated terminal diarylamino groups (4e–4f). The significant Stokes shift (ca. 170 nm for 4a–4d) reveals that the 3,3A-diphenyl-2,2A-bithiophene linkage may be highly twisted in the ground state. The emission maxima with relatively long wavelength could be attributed to the relaxation from an excited state with a more coplanar conformation. The influence of phenyl substituents of the bithiophene linkage on the photophysical properties are demonstrated by a comparison of the UV-Vis and photoluminescent spectra of 4b, 4e, and 5 (Fig. 1). Bis(triarylamine) 5 exhibited a sharper, red-shifted absorption and a blue-shifted emission compared to that of 4b. The longer emission wavelength of 4b further confirms a more conjugated excited state. Compounds 4a–4f exhibited only quasi-reversible anodic oxidation (Table 1). The onset of oxidation and Epa (V vs. Ag/ AgCl) varied with the nature of the terminal diarylamino groups. Differing from the conventional bis(triarylamines),13 only one redox couple (Epa 550 mV, Epc 430 mV vs. Ag/AgCl) was detected for 4a and 4c. Coulometry in a thin layer cell showed the redox process of 4a to be a two-electron oxidation. The lack of co-planarity of the central 3,3A-diphenyl-2,2Abithiophene linkage prevents the extension of p-conjugation along the molecular axis. The rate of second oxidation in 4a may be faster than the conformational change reaching a more coplanar conformation. Therefore, the first radical cation can not efficiently delocalise in the whole molecule, the two triarylamine systems behave independently but are oxidized simultaneously without any communication. Fig. 2 shows a comparison of the cyclic voltammogram of 4b, 4e, and 5. Two partially resolved redox processes (Epa 545 mV, Epc 455 mV and Epa 650 mV, Epc 570 mV vs. Ag/AgCl) of 4e were detected, which were assigned to be a two-step one-electron redox couple corresponding to removal of an electron from each triarylamine system. The redox potential difference (110 mV) of 4e indicates that the second oxidation could occur via a radical cation with more coplanar conformation. 4b, 4d, and 4f showed similar redox behaviour but were less resolved when compared with
that of 4e. The model compound 5 exhibited two well-resolved redox couples (Epa 460 mV, Epc 385 mV and Epa 695 mV, Epc 610 mV vs. Ag/AgCl). The lower oxidation onset and larger potential difference (230 mV) reveal that the the central bithiophene linkage is in a coplanar conformation. All the compounds in this study exhibited an amorphous nature evidenced by the presence of the glass transition temperature (Table 1). The asymmetric diarylamino substituent and the diphenyl substituted central bithiophene linkage significantly contribute to the high Tgs. Compounds (4d, 4e, 4f) with higher molecular weight aryl groups showed higher Tgs compared to that of 4a, 4b and 4c bearing a lower molecular weight aryl group in the terminal diarylamino substituents. In summary, we have successfully established an efficient method for the synthesis of a new class of bis(triarylamines) bearing 3,3A-diphenyl-2,2A-bithiophene as a central linkage. The introduction of phenyl substituents on the central linkage apart from improving the morphological stability, twists the conformation of the central bithiophene linkage in the ground state, which results in interesting photophysical and electrochemical properties. Further studies on the modification of the central bithiophene linkage by introducing bulkier aryl groups and their applications in OLED are under way and will be reported in due course. We thank the National Science Council of Taiwan for providing financial support (NSC-89-2113-M002-053).
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